The present invention relates to a DC power supply circuit for an AC adapter and, more particularly, to a DC power supply circuit enabled to suppress rise of an output voltage thereof by feeding electric current through a bleeder circuit when the output voltage thereof exceeds a preset voltage.
When an apparatus using a DC source, such as a battery, as an operating power supply is operated by power supplied from a commercial power supply, or when an apparatus needing to be supplied with DC current from an external source so as to reduce the size thereof is operated, an AC adapter for rectifying and smoothing voltage signals supplied from the commercial power supply and for outputting resultant voltage signals is used. This AC adapter is configured in such a way as to have a power supply transformer, a rectifier diode, and a smoothing capacitor. Thus, an output voltage thereof changes according to a variation in a load current or in a voltage of a commercial power supply.
The aforementioned prior art for preventing change in the output voltage is proposed in the Unexamined Japanese Utility Model Publication No. Hei 7-23993. That is, according to this prior art, a power supply transformer and a rectifier smoothing circuit are provided in an AC adapter. Moreover, the AC adapter has a constant voltage circuit, to which an output of the rectifier smoothing circuit is led, and sends an output of this constant voltage circuit to an external circuit. Therefore, even when a change in the output voltage of the rectifier smoothing circuit occurs owing to a variation in the voltage of a commercial power supply or in the load current, a DC output sent to the external circuit is stabilized at a constant voltage (incidentally, this prior art will be hereunder referred to as the "first prior art").
Further, generally, the minimum output voltage and the maximum output voltage of an AC adapter are prescribed according to specifications determined correspondingly thereto. Assuming now that the voltage inputted from the commmercial power supply is constant, when the output current increases, the output voltage lowers, as indicated by a line 51 or 52 in FIG. 5. Furthermore, the ratio of a decrease in the output voltage to an increase in the output current increases with decrease in size of the power supply transformer to be used. Therefore, in the case that the power supply transformer of the AC adapter, which is adapted in such a manner as to have an output voltage V1 when an output current is I2, is a small one, when the output current reaches 0, the output voltage rises to the voltage V3. Conversely, in the case of using a large power supply transformer, even when the output current is 0, the output voltage rises only to the voltage V2. Thus, a large power supply transformer is necessary in the case that the output voltage is the voltage V1 when the output current is I2, and that the output voltage is equal to or less than the voltage V2 when the output current is 0. This results in the rise of the cost of the power supply transformer and in the increase in the size of the AC adapter.
As the prior art for avoiding the increase in the size of the power supply transformer, there has been proposed a device configured so that a bleeder resistor is connected in parallel to the smoothing capacitor. That is, in the case that the bleeder resistor is connected in parallel to the smoothing capacitor, even when the output current is 0, electric current flows through the bleeder resistor. Thus, a load current of the rectifier smoothing circuit. Therefore, even when the output current is 0, rise in the output voltage is prevented owing to the effects of electric current flowing through the bleeder resistor. As a result, in the case that a small-sized power supply transformer is used in an AC adapter adapted to output voltage V1 when the output current is I2, the output voltage at the time, at which the output current is 0, is restrained in such a manner as to be equal to or less than the voltage V2 (hereunder, this prior art will be referred as the "second prior art").
However, in the case of using the first prior art, there is the need for providing the constant voltage circuit in the AC adapter. Thus, the circuit configuration of the AC adapter becomes complex. Moreover, the number of components to be used becomes large. These result in the large shape, complex manufacturing process, and high component cost of the AC adapter.
On the other hand, the second prior art can avoid the aforementioned problem. However, even when this prior art is used, the following problems arise. That is, when viewed from the power supply transformer side to the output side, electric current flows in a load device of the AC adapter. Moreover, electric current flows in the bleeder resistor. Thus, the output voltage of the power supply transformer increases. Furthermore, the heating value of the power supply transformer increases. Additionally, electric current always flows in the bleeder resistor, so that the bleeder resistor generates a large amount of heat. As a result, the rise in temperature of the AC adapter increases. Consequently, dangerous situations, such as combustion, are liable to occur.